[Seaside] Server-Timing filter goodie

[Esteban Maringolo]

Hi People,

I implemented a small request filter to support the Server-Timing
header (and web development tools integration).

See more here:
https://github.com/eMaringolo/seaside-server-timing

Note: I think there is a bug in the conversion of the headers from a
WAResponse to ZnResponse in ZnZincServerAdaptor, since if there are
several headers with the same name in aWAResponse, but only the last
one remains in ZnResponse, because it does an `at:put` instead of
adding several headers or merging them all together (as with the
'Set-Cookie' header).

By doing this I also learned that there is a `Trailer` header, that
allows you to add a header that behaves like a footer in a chunked
response. This would be useful to track some performance indicators at
the very end of the process. But I don't know if this Trailer header
is currently supported.

Regards!

Esteban A. Maringolo